I have had many placements where I didn't feel a true bond. Other placements I have felt a bond with from the get-go. It is totally normal.
I second this.
Some of my fosters I had to "fake it" the entire time. Things just never clicked for whatever reason.
But that's ok!
And it certainly doesn't mean you should stop fostering, unless of course it's something you don't want to do anymore. It is a good thing either way you decide to go, when a foster parent can be honest about those feelings. You don't love and fawn over every adult you meet do you? So why should it be the same for children? I think that expectation is there, and it's surprising as all get out when that turns out not to be the case with ever kiddo.
I'd suggest you shoot for a different age range and see how that goes. Would you be up for caring for an infant or pre-teen? Perhaps even older? You've mentioned caring for ages 2 to 8 I believe, maybe dabble above or below that and see if that changes anything for you